“It would be very difficult to achieve in the best of circumstances, and even more difficult under the policies that Senator McCain has proposed,” said Robert L. Bixby, executive director of the Concord Coalition, a nonpartisan budget watchdog group.

Mr. McCain, the presumptive Republican presidential nominee, is proposing billions of dollars in tax cuts. But advisers to Mr. McCain said those costs would be more than offset by savings from slower growth in spending.

In his proposal, Mr. McCain said he would hold overall spending growth to 2.4 percent a year. That is a tall order because federal spending has been growing an average of more than 6 percent a year in the last five years.

Mr. McCain said he would also slow the growth of Social Security, Medicare and Medicaid, and fiscal experts agree that he would need to do that to achieve his goal. But Mr. McCain did not give details of how he would alter those benefit programs, which have powerful constituencies, including older Americans, a huge health care industry and state and local government officials.

A longtime foe of pet projects known as earmarks, Mr. McCain said he would stop such spending. The Bush White House says earmarks this year total $17 billion, a comparatively small share of a $2.9 trillion budget.

Mr. McCain proposed a one-year freeze in most domestic spending subject to annual appropriations, “to allow for a comprehensive review.” This proposal would affect education, scientific research, law enforcement and scores of other programs.

Mr. Bush’s battles with Congress suggest it would be extremely difficult for Mr. McCain to win approval for such a freeze.

Mr. McCain said he was counting on “rapid economic growth” to help reduce the deficit. While a growing economy generates additional revenue, several of Mr. McCain’s tax proposals would be costly, experts said.

He would “phase out and eliminate” a provision of the tax code known as the alternative minimum tax, which has ensnared a growing number of middle-class Americans in recent years.

By his own account, repealing this tax “will save middle-class families nearly $60 billion in a single year.” That is $60 billion that would presumably not be available to the Treasury.

Mr. McCain also wants to extend many of the Bush tax cuts, scheduled to expire by Jan. 1, 2011. That could reduce tax collections below the levels assumed under current law, and it could widen the deficit, many economists said.

In January, the Congressional Budget Office estimated that extending the Bush tax cuts would cost more than $700 billion in the next five years.

Since January, the economy has been weaker than expected, making the goal of a balanced budget more difficult to achieve. The budget deficit in the current fiscal year is running much higher than in the previous year.

Other McCain proposals, like doubling the personal tax exemption for dependents and cutting the corporate income tax rate, would also reduce revenues, economists said.

C. Eugene Steuerle of the Urban Institute, who worked in the Reagan administration, said Mr. McCain “may well be committed to balancing the budget in five years, but does not tell you how he would reach that goal.”

J. Bradford DeLong, a professor of economics at the University of California, Berkeley, who worked at the Treasury under President Bill Clinton, said, “Senator McCain and his advisers want to claim they will balance the budget by 2013, but they have given us no clue and no plan to meet all the commitments he has made and still get there.”

On the other hand, history shows the deficit sometimes shrinks faster than experts expect.

That happened in 1998 in the Clinton administration, when the government ran a surplus for the first time in nearly three decades. And Mr. Bush cut the deficit in half faster than he or many fiscal experts had predicted.

“It would be very difficult to achieve in the best of circumstances, and even more difficult under the policies that Senator McCain has proposed,” said Robert L. Bixby, executive director of the Concord Coalition, a nonpartisan budget watchdog group.

Mr. McCain, the presumptive Republican presidential nominee, is proposing billions of dollars in tax cuts. But advisers to Mr. McCain said those costs would be more than offset by savings from slower growth in spending.

In his proposal, Mr. McCain said he would hold overall spending growth to 2.4 percent a year. That is a tall order because federal spending has been growing an average of more than 6 percent a year in the last five years.

Mr. McCain said he would also slow the growth of Social Security, Medicare and Medicaid, and fiscal experts agree that he would need to do that to achieve his goal. But Mr. McCain did not give details of how he would alter those benefit programs, which have powerful constituencies, including older Americans, a huge health care industry and state and local government officials.

A longtime foe of pet projects known as earmarks, Mr. McCain said he would stop such spending. The Bush White House says earmarks this year total $17 billion, a comparatively small share of a $2.9 trillion budget.

Mr. McCain proposed a one-year freeze in most domestic spending subject to annual appropriations, “to allow for a comprehensive review.” This proposal would affect education, scientific research, law enforcement and scores of other programs.

Mr. Bush’s battles with Congress suggest it would be extremely difficult for Mr. McCain to win approval for such a freeze.

Mr. McCain said he was counting on “rapid economic growth” to help reduce the deficit. While a growing economy generates additional revenue, several of Mr. McCain’s tax proposals would be costly, experts said.

He would “phase out and eliminate” a provision of the tax code known as the alternative minimum tax, which has ensnared a growing number of middle-class Americans in recent years.

By his own account, repealing this tax “will save middle-class families nearly $60 billion in a single year.” That is $60 billion that would presumably not be available to the Treasury.

Mr. McCain also wants to extend many of the Bush tax cuts, scheduled to expire by Jan. 1, 2011. That could reduce tax collections below the levels assumed under current law, and it could widen the deficit, many economists said.

In January, the Congressional Budget Office estimated that extending the Bush tax cuts would cost more than $700 billion in the next five years.

Since January, the economy has been weaker than expected, making the goal of a balanced budget more difficult to achieve. The budget deficit in the current fiscal year is running much higher than in the previous year.

Other McCain proposals, like doubling the personal tax exemption for dependents and cutting the corporate income tax rate, would also reduce revenues, economists said.

C. Eugene Steuerle of the Urban Institute, who worked in the Reagan administration, said Mr. McCain “may well be committed to balancing the budget in five years, but does not tell you how he would reach that goal.”

J. Bradford DeLong, a professor of economics at the University of California, Berkeley, who worked at the Treasury under President Bill Clinton, said, “Senator McCain and his advisers want to claim they will balance the budget by 2013, but they have given us no clue and no plan to meet all the commitments he has made and still get there.”

On the other hand, history shows the deficit sometimes shrinks faster than experts expect.

That happened in 1998 in the Clinton administration, when the government ran a surplus for the first time in nearly three decades. And Mr. Bush cut the deficit in half faster than he or many fiscal experts had predicted.